Adulticiding

Adulticiding, the control of adult mosquitoes, is the most common form of mosquito control used today. This process can be achieved through aerial or ground application of insecticides designed to control adult mosquitoes. The Collier Mosquito Control District (CMCD) uses aerial adulticiding as the major control method. This is due primarily to the fact that CMCD is located adjacent to the Florida Everglades and the 10,000 Islands area which produce huge numbers of salt marsh mosquitoes that migrate into the District.

The aerial application of adulticides allows the district to treat areas that are either inaccessible or too large to be treated effectively from the ground. Aerial application treats mosquitoes while they are in flight and vulnerable to the application. This allows mosquito control operations to treat large populations of mosquitoes at once. However, there are several important factors that have to be considered in order for mosquito control to be most efficient, such as species susceptibility to insecticide, whether the target species is diurnal (active in the day time) or crespuscular (active after dusk), the proper dosage rate of the insecticide, and the environmental conditions at the time of application. Without this information, aerial application of adulticide would not be effective.

There are two common forms of adulticiding; thermal fogging, and ultra low volume (ULV) application. Thermal fogging uses the exhaust heat from an aircraft's engine to atomize diluted mixtures of petroleum and insecticide. Ultra low volume application uses less insecticide than thermal fogging. This process creates fine droplets of pesticide as it flows through hydraulic nozzles in conjunction with the high-speed air stream created by the aircraft. The ULV application has less effect on non-target species because the amount of insecticide is reduced and the petroleum is eliminated completely. The CMCD began phasing out thermal fogging in the early 1990's. Since then, the District focuses on ULV application using Baytex and Dibrom as insecticides.

All fixed wing applications are currently made at night, usually at 2 a.m., because more mosquitoes are active at this time and to take advantage of better environmental conditions. At this time of the night the earth has cooled so there are no thermal convection currents rising from the ground, which could prevent the insecticide being applied from reaching the target area. The wind has generally stabilized in both directions, usually from the east, and speed, generally under 10 m.p.h. at ground level. In addition, it is too early in the morning for ground fog, which can limit aircraft operations, to form.

The CMCD's practice of adulticiding has changed dramatically over the past decade. For example, in 1989 it took five fixed wing aircraft over three days to treat 85 sq. miles (using thermal fogging). This used 170 gallons of insecticide and 8,500 gallons of petroleum. The mixture was sprayed out at 20 oz. per acre during the first hours after sunrise. Not only did this method affect non-target species because the amounts of pesticide petroleum were so great, but it was less effective than current practices because fewer mosquitoes are active during the day. In addition, this cost the district $65,000.

In 1995 it only took two fixed wing aircraft one night to spray the same area, applying only 0.66 oz. of insecticide per acre. The treatment was done at night so more mosquitoes were controlled, and fewer non-target species affected. The amount of insecticide used was also drastically reduced. This application cost $43,000.

Today, it takes one fixed wing aircraft less than three hours to treat the same area, and uses 0.21 oz of insecticide per acre when taking drift into account. This amounts to $9,500 for application. All treatments are applied before or at dawn, so fewer non-target species are affected. However, the CMCD is still focusing on improving the level of mosquito control while reducing potential environmental effects and reducing the cost of control. The District has designed a new air assist spray system that is installed on all the fixed wing aircraft and one helicopter. The air assist spray system is new to the helicopter and will be used for the first time during the summer of 2002.

How do adulticides kill mosquitoes? 
Most adulticides work by disrupting the nervous system of a mosquito. The mosquito’s nervous system consists of a brain and neurons. Neurons are nerve cells that transmit messages to organs in the body. Adulticides stop the message from being correctly sent through the neuron which causes the nervous system to overload and stop functioning. Without the nervous system functioning properly, the insect cannot survive, and therefore dies.

The nerve cells are stimulated by electrical impulses that travel between neurons in a space called a synapse. The electrical impulses given to the neuron are commands that tell the neuron to do something, or to stop doing something. The signals that stimulate the neuron to do something are carried by a chemical called acetylcholine. The stimulating signals are stopped by a chemical called acetylcholinesterase, which is an enzyme. The acetylcholinesterase breaks down the acetylcholine, which stops the message transmission. These chemical reactions occur constantly, at a very fast rate, with acetylcholine stimulating the neuron and acetylcholinesterase ending that signal. A muscle contraction is an example of an activity produced by this chemical reaction. The brain sends a signal, or nerve impulse, with acetylcholine through the synapse between the nerve and the muscle, stimulating the muscle to contract. After the movement is accomplished, acetylcholinesterase is released. This breaks down the acetylcholine and stops the stimulation of the muscle to contract.

When an adulticide penetrates the skin of a mosquito, it travels to the synapses in the nervous system and prevents the acetlycholinesterase from breaking down the acetylcholine. For this reason, adulticides are referred to as cholinesterase inhibitors. This means that the signals being sent to the nerves are not stopped. If these signals are going on constantly at a very fast rate, and the acetylcholine is not being broken down, then there is a build up of acetylcholine in the nervous system. The excess acetylcholine overloads the nervous system and the mosquito dies. For example, the brain sends a message, through acetylcholine, for a muscle to contract. Once the muscle movement is complete, the adulticide stops the release of acetylcholinesterase. That means the muscle keeps getting the message to contract. This causes uncontrolled, rapid muscle twitching which eventually kills the mosquito.